A diesel particulate filter (DPF) is a gas treatment device that is commonly arranged within an exhaust gas stream of an internal combustion engine to trap particulates present in the exhaust gas. A DPF may include a cylindrical metal housing wrapped around a cylindrical ceramic filter element. A resilient mat may be compressed between the outer wall of the filter element and the inner wall of the metal housing. Because the mat is resilient and compressed around the filter element by the housing wall, the mat may help secure the filter element within the housing while reducing vibratory effects between the housing and the filter. Typically, a pliable annular retention gasket and a rigid annular retention ring are secured within the housing at each axial end of the filter to further secure the filter element within the housing.
During a DPF assembly process (the “canning” process), a resilient mat may be wrapped around a ceramic filter. Then a sheet metal housing shell may be tightly wrapped around the mat and the filter to compress the mat against the filter. When a desired dimensional relationship is achieved between the shell, the mat, and the filter, the shell may be welded closed. A pliable annular retention gasket may be placed at each axial end of the filter, and a rigid annular retention ring may be secured within the housing adjacent each retention gasket.
There are at least two methods for creating a desired dimensional relationship between the shell, the mat, and the filter during the canning process. The first method may be referred to as a “can-to-size” method, during which a sheet metal shell is wrapped in a tubular shape around the filter and the mat until the metal shell reaches a target diameter. After a target diameter is reached, the shell is welded closed. The can-to-size method ensures a desired shell diameter is accomplished. Thus, single-size retention gaskets and rings may typically be used within the housing when a can-to-size method is used. However, since the diameter of a filter element and the initial thickness of a resilient mat may vary due to common part-to-part dimensional supply variation, the can-to-size method may inconsistently compress a mat around a filter element. For example, if a manufacturer uses mats with a supply variation of +/−1 mm diameter and filters with a supply variation of +/−2 mm diameter, the diameter of a mat-filter assembly could vary by as much as 8 mm. Since the can-to-size method produces housings having the same diameter, it should be appreciated that mat-filter assemblies with smaller diameters (due to supply variations) may cause mats to be compressed within the housings less than mat-filter assemblies having larger diameters. Thus, multiple DPF's produced using the same can-to-size method may exhibit inconsistent mat compression values. Moreover, the more a mat is compressed around a filter, the more axial force the filter can withstand before being dislodged from the grip of a shell and mat. Therefore, dimensional variations in supply mats and filters may result in varying degrees of “holding forces” being exerted on filters in DPF's assembled using a can-to-size method.
A second method for determining a desired relationship between the shell, the mat, and the filter during a canning process is the “can-to-force” method. During this method, a sheet metal shell is wrapped in a tubular shape around the filter and the mat until the mat is compressed a target amount between the filter and the metal shell. The can-to-force method ensures that a desired holding force is exerted by the housing and mat on the filter. However, since the diameter of a filter element and the initial thickness of a resilient mat may vary due to common part-to-part dimensional supply variation, the can-to-force method may produce inconsistent housing diameters. Thus, different size retention rings may be used within different housings to accommodate the varying housing diameters produced with a can-to-force assembly method.
Some manufacturers may consider using a can-to-size assembly process with over-sized mats so that variations in holding force resulting from varying component dimensions are less significant. Other manufacturers may consider using a can-to-force assembly process while stocking differently-sized retention rings.
Prior assembly processes may be improved, for example, by permitting use of optimally sized mats and filters while reducing component stocking requirements.
The present invention is directed to overcome or improve one or more disadvantages associated with prior apparatus and methods for treating gases.